Solution: Total number of outcomes = 12
P(x) = x/12
When 6 more black balls are put in the box, total number of balls = 12 + 6 = 18
Number of black balls = 6 + x
P(black) = (x+6)/18
As per question;
(x+6)/18 = 2*x/12
or, x+6 = 3x
or, 2x = 6
or, x = 3